Automatic and semiautomatic telephone exchange system



April 12, 1932- Y I G. Dl-:AKIN ET Al. 1,853,962n

AUTOMATIC AND SEMIAUTOMATI'G TELEPHONE EXCHANGE SYSTEM Filed May 1s, 195o 14 Sheets-sheet 1 ATTORN EY.

Apri112,193z. Q DEAKIN ETAL 1,853,962I

AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM ATTORN EY April 12, 1932.y G. DEAKIN ET AL AUTOMATIC AND SEMIAUTOMATICl rIAILEPIIONE EXCHANGE 'SYSTEM Filed May 13, 1950 14 Sheet-Sheet 5 FIG. 3

' I lNvENToRs a? 5000412059004, ,10M/wsrf@ 70K fffmw.

ATTORNEYl u April 12, 1932. G. DEAKlN ET AL 1,853,962

AUTOMATIC AND SEMIAUTOMATIC.TELEPHONE EXCHANGE SYSTEM A Filed May 15, 195ov 14 sheets-sheet 4 FIG. 4

ATTORN EY April 12, 1932. G. Dl-:AKIN ET AL 1,853,962

AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 13, 1950 14 sheets-sheet 5 NTORS ATTORNEY April 12, 1932. G, DEAKIN ET AL 1,853,962

AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 15, 195o `14 sheets-'sheet 6 ATTORN EYl 14 Sheets-Sheet 7 LUMN G. DEAKIN ET AL Filed May 15, 1930 AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM April 12, 1932.

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ATTORNEY AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 15, 1950 14 Sheets-Sheet 8 INVENTORS @584m 054km/ hD/1 ,00L mfom/5K); 05654550 j', foam/ea new, ,foM/n//srffm/g ATTORNEY April 12, 1932- G. DEAKIN ET AL v 1,853,962

AUTOMATIC'AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 15, 195o A14 sheets-sheet 9 FIG. 9

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' ATTORNEY April 12, 1932. G. Dr-:AKIN ET AL AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 13, 1930 14 Sheets-'Sheet 10 INVENTORS 50004:?0 ffl/All @y ATTORNEY III-II' April 12, 1932 G. DEAKIN ET AL 1,853,962

AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM.

Filed May 15. 1950 14 SheetsLSheet ll zFr @7 pWZM-f April 12, 1932. G. DEAKIN ET AL 1,853,962

A AUTOMATIC AND SEMIAUTMATIC l'ELE PONE,EXCHANGE SYSTEM Filed May 1:5, '1930 1,4 sheets-Sheet 12 WCD OTK I y fname@ FewMMMM//srm me ATTORNEY April 12, 1932. G. DEAKIN ET AL AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 13, 1930 14 Sheets-Sheet 15 in? l" FIG. I4

ATTORNEY April 12, 1932. G. DEAKIN ET AL.

AUTOMATIC ANDV SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM Filed May 13, 195o 14 sheets-Sheet, 14

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Patented Apr. l2, 1932 UNITED STATES PATENT oFFlcE GERALD DEAKIN, OF NEW YORK, N. Y., AND LIPA POLINKOWSKY, DECEASED, LATE F PARIS, FRANCE, BY EDOUARD FRUM, ADMINISTRATOR, OF PARIS, FRANCE, AS-

SIGNCRS T0 INTERNATIONAL STANDARD ELECTRIC CORPORATION,

Yoax, N. Y.

OFNEW AUTOMATIC AND SEMIAUTOMATIC TELEPHONE EXCHANGE SYSTEM .Application led May 18, 1930, Serial No. 452,083, and in Great Britain May 14, 1929.

This invention relates to machine switching telephone systems and more particularly to systems wherein in addition to the ordinary service provided for the interconnection of the subscribers lines, means` is provided for extending connection to subscribers lines from a toll operators position vor a wire chiefs testing board. l

A particular object of the invention is the 1o provision of such facilities in an automatic or semi-automatic telephone system of the kind in which the conversation switches at the numerical switching stages are operated under the control of control circuits each common to a plurality of conversation switches but individual to one switching stage.`

According to the invention in a system of the above type, the same final conversation switches are used for extending ordinary calls and toll calls means being provided in the common control circuits associated with the final conversation switches whereby differentoperations of the latter for completing the two types of connection are brought about.

According to a further feature of the invention in a system of the above kind, conversation switches are provided for setting up both ordinary connections and toll connections, common control circuits associated therewith being provided with means for eX- ercising a discrimination whereby in thecase of an ordinary connection ringing current is automatically applied to a wanted line but in the case of a toll call a connection is com'- pleted to a wanted line without such automatic application of ringing current.

According to st1l1 a further feature of the invention in an automatic or semi-automatic telephone system in which means is provided' for settting up toll calls and in which the These and other features specified in the appended claims will be better understood from they following description taken in conjunction with fthe accompanying drawings in which the invention is described as applied to a system using single motion power driven switches for both the conversation switches and vthe control circuit steering switches. l

n Referring now to the accompanying drawlngs: c

Fig. 1 shows a subscribers line and starting circuit and a first line finder'.

Fig. 2 shows a second line finder circuit.

Fig. 3 shows a first group finder, circuit. lig. 4 shows asecond group finder cir- Clll 1 Fig. 5 shows a final finder circuit.

. Fig. 6 shows a first group finder control circuit.

Fig. -7 shows a second group finder controlv circuit.

Fig. 8 shows a final finder control circuit.

Fig. 9 shows a register circuit including a register iinder.

Fig. 10 shows an incoming two-wire junction relay set.

Fig. 11 shows a special service operators position.

Fig. 12 shows a toll junction' equipment and (in the left-hand top corner) a. toll board cord circuit.

Fig. 13 shows a toll offering and wire chief testing junction equipment and (in the lefthand top corner) a toll board cord circuit and wire chiefs position in skeleton.

Fig.4 14 shows a `toll offering and wire chiefs testing final finder circuit.v

Fig. 15 is a layout diagram of a typical system. A general with reference to Fig. 15.

description will first be given l Wlien the telephone is removedfrorn the hook at substation S the calling line is extended over first and second line nders LFl,

LFE?, to a first group finder lGF having an idle associated control circuit lCF. Im-

mediately thereafter the line finder circuit comprisino the second line finder LF2 and group findzr lGF is attached to an idle register circuit RC over a register finder PR,

whereupon dialling tone is sent to the calling art I Tlire subscriber dials in the usual way. Ihe first digit dialled advances the first digit switch 1n the register circuit to the corresponding position. When the first di it has been received a signal is sent forwar from the register circuit to cause the first group finder switch lGF associated with the line finder LF2 to become connected with its associated control circuit the finder switch lCF of which is set in rotation and hunts until it finds the first group finder switch lGF. The control finder lCF is then further advanced to a position corresponding to the first digit dialled in which position 1t marks the wanted group of trunks outgoing from the bank of the switch lGF: during the lastmentioned movement revertive impulses are sent back from the control finder ICF to count out the first digit switch in the register circuit RC. The first group finder4 lGF is now caused to hunt for a free trunk extending to a second group finder QGF the associated control circuit of which is idle, in the group determined by the setting of the control finder ICF. It should be noted that the trunks from the arcs of thegroup finder switches need not be arrange in definite orderand further the groups may not contain the saine number of trunks. Whenever it is found necessary to increase the number of trunks in a particular group this can be done without disturbing the existing connections, the additional trunks being connected to any spare positions in the arcs.

The further extension of the connection .des pends upon the type of call which is being set up. We will assume for the time being that the call in progress is a local one. When the second digit has been dialled, a signal is again sent forward from the register circuit- RC to cause the second group finder QGF which has been taken for use to become attached to its control circuit, the finder, QCF of which is set in rotation and hunts until it finds the en aged second group finder 2G-F. The Control nder 20F is then further advanced to a position corresponding to the second dialled digit, to mark a wanted roup of trunks in the bank of the finder QF, during this movement revertive impulses being sent back to the register circuit to count out the second digit switch therein. The second group finder QGF is now caused to hunt in the wanted group for a free trunk to an idle final finder FF the associated control circuit of which is also idle.

When the third cr tens digit and at least one impulse of the fourth or units digit have been received by the register circuit, the control circuit of the final finder FF is taken into use and its finder FCF hunts until it finds the final finder FF. The control finder FCF then advances further to a position corresponding to the tens digit during this movement revertive impulses heilig sent back to count out the tens digit switch. The final finder FF now advances to the wanted group after which the control finder F CF continues to advance to a position corresponding to the wanted line; during this movement the final finder FF follows the control finder FCF and a revertive impulse is sent to the units digit switch for each/position through which the switch FCF is advanced.

In addition to controlling tens and units selection the final control circuit determines tional contact over which an engaged poten.-

tial is applied to a corresponding terminal in the arc of the control finder FCF when the line is busy. In case the called number corresponds to a P. B. X. group and the first line is found busy, the control finder F CF causes the advance of the final finder FF until a free line is found. In case all the lines of the group are found busy the usual busy tone is sent to the calling line.

It should be noted that the control circuits are not engaged until the register circuit is ready to receive theV corresponding revertive impulses. By this means the control circuits are held in use for the shortest possible time. In case all the trunks of a wanted group are busy the rQup finder switches are arranged tgcontiiue hunting for a predetermined time, say 10 seconds. If a free trunk is not found within this time the register circuit frees the control circuit and applies busy tone to the calling lines.

It will also be seen that a control circuit finder has a Idual function. It first advances from a Anormal position and causes the attachment of a predetermined trunk and an associated group or final finder switch to the control circuit and afterwards it controls the setting of the grouper final finder. For eX- ample, there-are usually ten trunks and their associated finders in a group served-by one control circuit. If trunk No. 6 has been selected from a. preceding switching stage, the control circuit finder advances and when position six is reached this'trunk is attached. The control finder then continues to advance and sends back the revertive impulses.

All the switches in the talking circuit are of the non-homing type and on release of the connection remain in the posit-ions to which they were last set. f

The re ister circuit RC is arranged to handle two-c igit special service calls for which the numbers 01 02 etc., are reserved. Such calls are extended to an operators position OP over a regular first group finder, such as lGF, and a special second group finder such as SGF controlled by a control finder SCF. The transmission bridge at the line finder circuit LF2, lGF is removed on such calls and the calling line is connected lnetallically to the manual cord circuit-from which battery for talking is supplied. Such a connection remains under the control of the operator and the automatic subscriber cannot release until theoperator takes down the connection.

False calls are also Vautomatically extended to a similar operators position over two group selecting stages. The number O0 is assigned to false calls the register circuit being automatically set to these digits by grounded lines or failures to dial Within 8O seconds. When a false call is extended the current bridge is automatically removed from comes connected to a regular register circuit RC. The group finders TGF are similar to the regular second group finders QGF. The same second group finders QGF and final finders FF are used for extending lthese calls, but When taken into use from a toll junction circuit TR the register circuit RC is arranged to exercise a discrimination whereby the final finder FF suppresses ringing current so that the calling of the Wanted party is entirely under the control of the toll operator. If the Wanted line is engaged the toll operator receives the regular busy tone.

Toll offering boards TOB and Wire chief testing boards WCB are provided for obtaining access to junctions incorporating relay groups TOR and terminating in first group finders TVGF. 7hen a junction is taken for use 'it becomes connected to a regular register circuit RC. A special train of first `and second group and 'naal finder switches Two-Wire junctions IJ fromdistant offices terminate in relay groups IJ R and incoming group finders JGF controlled by control circuits comprising control finders JCF.

These group finders and control circuits are identical with the local group finders QGF and their control finders QCF. The relay group IJR incorporates a transmission bridge which is arranged to be cut out and a clean metallic circuit substituted therefor' during the setting up of a connection.

Typical calls Will now b-e traced out in detail Local @all On a subscriber removin his receiver to make a call the line relay Fig. l is energized over the calling loop. Relay Lr places a battery potential on the first li-ne finder test multiple d and closes a circuit for a common starting relay Fsr which energizes starting relays AS1. Relays A37 close circuits for the clutch magnets PF, of all free first line finders in series With relay LgL via contacts 1125712, LWS. 'If there are no free finders in the group relay Lgr remains deenergized and a guard lamp LGL lights.

When a finder, say the one illustrated, reaches the calling line, test relay Ltr operates over its high resistance right-hand Windin g. opens at contacts Ltr the circuit of magnet PF and at contacts Ltrl closes a circuit for relay Liz/l' over its right-hand Winding and the common starting relay C81 of a subgroup of second line finders. Relay Ltr also connects battery to the second line finder test multiple d. Relay LW at contacts Lwl connects the low-resistance left-hand winding of relay Ltr in parallel With its high-resistance Winding to cause the relapse of relay 7" f1 Which de-energizes the relays AS1* to ai t the remaining first line finders. The group of second line finders is divided into four subgroups, the arrangement being that only the free finders of' one sub-group are set in rota-' tion for one originated call, but in the event of all the finders in this sub-group being engaged, the free finders of the next sub-group4 are set in rotation and so on:

The relay Csr' of the first sub-group energizes on auxiliary relay H81 whichl energizes one or more starting relays Bs?a of the sub-r group andprepares a circuit for the auxiliary relay Hair ofthe next sub-group. The relays B81 and H31 close arallel drlving circuits for thefree finders o the first sub-group over a common relay Sg1. If no finders are available in this sub-group the slow-acting relay A51 will be energized and close the circuit of the C81 relay and relay H8151 of the second sub-group in which free finders will be started by the energization of the B81 relays. lt will be easily understood from the circuit that if no finders are free in the second subgrou the third sub-group starting relays will Ee operated and so oI`1` until all the four sub-groups are in action. If no finder 1s free in the whole group, a supervisory lamp 4SGrL is lighted over a circuit extending over front contacts of the A61 relays of the last three sub-groups.

The pilot relay S91 is adjusted to operate over a single driving circuit. If desired 1t may be so adjusted that it does not operate unless two, three, etc. drivin c1rcu1ts are closed, i which case a secon sub-group 1s started if only two, three etc., finders are available in thev first sub-group.

In the second finder illustrated in Fig. 2

the driving magnet PL is energized in a circuit extending over contacts A11, F11, G13, and D12. When a second line finder has picked up the first line finder, the test relay A1 operates over its high-resistance left-hand Winding to ground at contacts 1Ec111 in the control circuit, Fig. 6 serving the first group finder, Fig.- 3 'linked to the second finder. If the control circuit, Fig. 6 is operatively associated with another. group finder, the relay 1Ec11will be energized as described subsequentl no circuit will exist for the test relay 1, and another second finder will pick up the engaged first finder.

Relay A1 opens the driving circuit at contacts A11 and closes the following circuit for relay C1 ground winding of relay S91, Fig. 1, contacts A12, right-hand winding of relay C1 and contacts D17 to battery. lRelay C1 at contacts C11 connects the low-resistance right-hand winding of relay A1 in parallel with its high-resistance Winding causing the relapse of relay CS1, Fig. 1 provided there is no other call existing at this instant in the same group. Relay C1 also at contacts C12 closes a circuit for relay B1 which energizes the relay D1 in a circuit over contacts B11, G17, F16: a locking circuit is now closed for relay C1 via contacts C13, E13 and lthe make before break contacts 1316-7. Relay D1 at i contacts D14 connects ground to the third wire c causing the l operation of the cut-off relay C01, Fig. 1, over the left-hand lowresistance winding of relay LL1 which remains operated in this circuit. Relay C01 disconnects the line relay L1 which opens the circuit of relay Lt1. Meanwhile the re- P lay C81 has de-energized and opened the cirfurther hunting.

Relay D1 at contacts D11, D13 connects` the supervisory relay S1 across the calling loop and at contacts D17- 6 connects battery via contacts C14 to the leads 1 and 2 to energize the register starting circuit relays A11, Fig. 9, and to apply test potential to the test contacts in the banks of the register finders. Relay B1 is now held over. contacts S12 of relay S1.

The register starting rela s A11 energize starting relays B11 which cllose circuits for the clutch magnets PR of register finders associated with free registers: in the register circuit shown this circuit extends from battery, over magnet PR, contacts G1141, E113,

B113, C111, to ground. The test relay C11 of the first finder` which reaches the engaged second finder circuit energizes over its lefthand high-resistance winding via contacts D111, normal contacts and brush a of the units register, normal contacts and brushes d of the remaining digit registers and ground, opens the circuit of magnet PR and closes a circuit for relay E11. Relay E11 shunts the high-resistance winding of relay C11 with its right-hand 'low-resistance winding atv contact E111, lights a busy lamp BL at contacts E111-2, opens a further point in the circuit of magnet PR at contact E113 and at contacts E116 and E117 connects dialling tone to the calling line. The dialling tone circuit extends from the right-hand winding of the tone transformer TC, small condenser MF1, contact R16, windings of relay P1, contacts E116, register finder wiper b, contact E12, Fig. 2, b wire bridge condenser, calling loop, a wire lbridge condenser, contact E11, register finder wiper a, Fig. 9, contact E117 back to the tone transformer winding. The circuit of the primary winding of the transformer T-C is completed via dial tone lead D, contacts F112, E114-3, G114 and the winding of magnet.

The calling party now dials the first ori thousands digit of the wanted number, thereby intermittently opening the circuit of relay S1, Fig. 2, which at each relapse closes an impulse circuit for relay I1 and H1, Fig. 9, via contacts S11 and E115. The relay B1, Fig. 2 is of the slow-releasing type and during impulsing remains operated. At each energization of the impulse relay H1 a circuit is Aclosed over contacts H12, J 13 for the stepping 'namens U1, windin of relay V1, brush b of rst digitre ister, contacts D116 to ground. Owing to t e high-resistance o relay E1 rela .V1 does not operate in this circuit. A loc ing circuit for these relays is' closed v'ia contacts U13 and D116.

Relay E1, Fig. 2, disconnects the dial tone,

` and at contacts E13 opens the locking lcircuit of relay C1, which removes the test potential from the contact engaged by brush d of the register finder, thereby de-energizing relay C11, Fig. 9. Relay E11- Fig. 9 however remains held via contacts U12. At the end of -the first impulse seriessrelay I1 relapses and When the subscriber sends the second series of-impulses thamagnet S102 of the second digit register responds and the register takes up a position corresponding to the digit dialled. On the cessation of these impulses relay L1 becomes energized and transfers the impulsing circuit to the magnet SW3 of the third digit register overcontacts T114, T16,

O16. The third series of impulses advancev this register and when the last impulse is received relay O1 energizes and transfers the impulsing circuit via contacts O15 to the magnet SW4 of the last digit register which responds to the last series of impulses.

On the response of -relay- J1 at the end of the first impulse series the following circuit is also closed for relay K1:-+ground, contacts J 16, G112 left-hand winding of relay K1 contacts F116 to battery: relay K1 locks over contacts K12, N11, E112-1 independently of contacts J16 and G0112.y Relay -K1 connects ground to the b wire of the fundamental circuit via contact K14, winding ofY the outstep- -ping relay P1, contacts E116, and the register iinder brush b, and connects battery to the a wire of the.,` fundamental circuit via a resistance, contacts K13 and the register nder brush a. j L 1 The connection of battery to the a wire I closes a circuit for the common starting relay A01, Fig. 6 of the first group finder control circuit via contacts C15, Fig. 2 and J212, f

Fig. `Sand lplaces a potential on the contact of the level wiped over by brush z' particular to the engaged first group finder.

It will be convenient here to describe the arrangement of the bank contacts of the group findr control circuit switches. vThese have 51 points or positions (the first being used asfahome position) andv live levels of contacts wiped over by brushes The ten contacts in the first ten positions in levels 'h and 1l are wired to the ten group finders usually controlled by one nontrol circuit.

oE-normal "are and The remaining' 40 points in these two levels are not used. Points 11 to 50 are used for the reverting control and in levels f and g. are divided into ten oups of four each. In

level g the second an third vcontacts of each group of four are strapped together andv grounded. The fourth contact of each group of four in level f is wired to the marking terminals d of 'its' corresponding group of ]unc tions v "outgoing from the group finder switches. The marking wires are connected to terminals f1, f2, f3 etc. on a small terminal strip from which j'umperin wires are taken to the group finders. T e 100 outgoing trunks, the normal capacity of the group finders, may be divided into Iten groups of different sizes, providing the total remains 100.' The d terminals Aof all the trunks of the first group are wired to the first terminal in level f of the control switch, all those of the second group to the second f terminal and so on. All the trunk groups may comprise 10 trunks each or some groups may be larger and others smaller. As the group finders have no home positions, the junctions in each group may be distributed in any order 'around the arc` so asto reduce hunting.

Where ten trunk groups are not required the finder arcs may be divided into fewer groups. For example, assuming av network involving an ultimate capacity of four oflices, the rst group finder' arc might be ,divided into 5 groups,say, one of 25 for yjunctionsfto the most distant oliice, 2 of 20 for junctions to theremaining distant oiiices, one of 25 for local calls, and one of 10 to a group of special service ilfders. The contacts in the level j :are grounded and are used for centering` the brushes.

Resuming now the operation-of the system,

closes a circuit over contacts F012 and B011 for the power magnet PC and the controlV nder switchv is rotated. A circuit is also closed for relays E01 which remove earth from the left-hand windings of the test relays A1, Fig. 2 of all the second linders linked with the first group finders served by the con trol circuit in order to prevent any of these second finders from picking up a first finder whilst the control circuitis in use. When the ate. Relays F01 and J 11 lock to ground atV contacts B11, Fig. 2, via contacts J113, J211,

and the. c wire, and at contacts J 111 connectsthe b Wire of the fundamental circuit to relay calling trunk is reached, the test relay B01' D01 of the control circuit. The outstepping relay P1 in the register Fig. 9 now energizes in series with relay D01, Fig. 6. Relay P1 closes at contacts P12 a circuit via contacts M14 for relay R1, which locks to groundover its contactsR14, and wiper of the first digit register and contacts E112- 1, providing that the switch is in one of the positions 1 to 9. This locking circuit isnot closed if the switch is in position 10. Relay R1 closes an obvious circuit for relay N1 and at contact R15 connects the outstepping relay P1 in a metallic lcircuit across the a and b wire yof the fundamental circuit. Relay B01 Fig. 6 now relapses bein unable to remain operated in series with re ay D01. Relay N1 at contacts N11 opens the locking circuit of relay K1. In the control circuit relay D01 closes the following circuit for thepower magnet PC of the control finder switch ground, off-normal contact N, contacts F011, D011, G013, winding of magnet PC to battery. The control switch new advances its brushes 011er the remainder of the ten calling junction positions and in its further advance each time the f brush g passes over a grounded/group contact the outstepping relay P1 of the register is short-circuited for an instant. At each relapse of relay P1 a circuit is closed via contact P11 and R14 for relay Q1 which closes a circuit via contact Q12 and brush f of the first digit register for the magnet SW1 thereof and locks up via contactQ11 and brush 0 of this register to the grounded interrupter contacts I of the magnet SW1, thus maintainin the circuit of magnet SW1 closed even i relay P1 should open its back contact before magnet SW1 is fully energized. In this manner the 'first digit register is advanced until it reaches position 10 in which the locking circuit of relay R1 is opened. At

the next de-energization of the outstepping relay P1, relay R1 vrelapses and at contact R15 opens the fundamental circuit. Relay N1 is now deprived of current, but being of the slow-to-release type remains on its front contact N12 long enough to cause the energization of relay S11 via contacts R11 and E112-1. Relay S11 closes the foilotving circuit for relays M1 and Q1:-ground contacts E111-2, N12, S111 brush 0 of first digit register, left hand winding of relay M1 and winding of relay Q1^to battery. Relay Q1 recloses the circuit of magnet SW1 which advances the first register to its normal position whilst relay M1' recloses the fundamental circuit at contact M15 and locks over its righthand winding via contacts F116, M13 and P12. If the first 'digit register is set by dialled impulses to position 10 the relay fR1 relapses on the rst de-energization of the outs'teppinxg relay P1 and the operation proceeds as described above.

During the opening of the fundamental circuit at the register between the relapse of relay R1 and the response of' relay M1, relay D01, Fig. 6, in the finder control circuit deenergizes and closes a circuit for Arela G01' extending from ground at off-normaly contacts N Via contacts F011, D012 and G011. Relay G01 locks over contacts G011-2 and connects ground via contacts G015-4, the lli h-resistance right-hand winding of test re ay H01, brush f and the engaged marking terminal,Y to all the d terminals of the group of vtrunks corresponding to the first digit dialled by the subscriber. The power magnet PJ of the group finder also receives ground over contacts G015, H011, and .1114, and the group finder rotates. As its brushes 0 and d are connected together over contacts J1`16, when'1a trunk in the wanted group is reached at which both the group finder and associated control. switch are idle the test relay II01 in the control circuit energizes from the battery connected to the 0 wire of the trunkv over contacts 2E011, Fig. 7 Relay H01' connects la low-resistance test relay S01' and its own low-resistance left-hand winding in parallel with its high-resistance right- -hand Winding thereby busying the seized ltrunk to other calls. Relay S01 closes a circuit via contacts S011 and J115 for the group finder relay J 21' which locks to the grpunded front contact B11 of relay B1 Fig. 2 in lthe line finder circuit. Relay J21 completes at contacts J213 and .T214 the through connection from the line finder circuit to the seized trunk and opens at contacts J 211 the locking circuit of relay J 11 which releasesthus disconnecting the group finder from the control circuit. Relay F01 is also deprived of.current when relay J 21 energizes, opens the holding circuit for relay G01 and closes a, restoring circuit for the control switch over the off-normal contact N and contactsF012 and B011. If the relay A01 is energized due to another trunk beingin a calling condition; the switch will continue rotating and the second trunk will become immediately' attached. .y `Returning now to the register, when the relay L1 energizes after the recording of the second or hundreds. digit, a further circuit is closed for relay K1 which extends from battery, left-hand Winding of relay Kr, contact L16,rbrush b of hundreds digit register, and contacts M11, N11, E112-1 to ground. Relay K1 re-connects battery to the a wire and energizes the starting relay 2A01,-Fig. 7 of the .control circuit of the second group finder,

and reconnects ground through the outstep ping relay P1 to the b wire. It will be seen `therefore that if a second group nder is taken into use before the hundreds digit vhas 

